Virtual machines allow multiple operating systems to be run simultaneously on the same computer hardware. This allows the sharing of the underlying physical machine resources (e.g., memory, I/O, etc.) between multiple operating systems (or instances of the same operating system). Virtual machines facilitate application provisioning, maintenance, high availability, and disaster recovery. The software layer providing the virtualization is typically called a virtual machine monitor or hypervisor. A hypervisor may run on bare hardware, or on top of an operating system.